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Seiche

Seiches are stationary oscillations in an enclosed or semi-enclosed basin. They manifest themselves as oscillations of the water level, whose amplitude and period vary depending on the configuration of the basin.

The amplitude of the seiches can vary considerably from one port to another. They occur in many ports, but are often not noticeable because they are of low amplitude. In France seiches of significant amplitude can occur in Brest, Le Conquet, Cherbourg and Royan. The largest seiches occur on Groix Island in Port-Tudy, where they can reach 1.7 metres.

Background

The word "seiche" is a local term used by boatmen from Lake Geneva to designate the slow oscillations of the water level, large enough to be observed without instruments.

In 1730, the Swiss engineer Duillier completed the description, indicating that seiches may be caused by the weather.

Seiches were measured in the lakes of Scotland in 1755 following the Lisbon earthquake. In Loch Lomond, the water level rose 76 centimetres in 5 minutes then dropped to the lowest summer tide level before returning to the maximum height.

After 1869, routine observations were made on Lake Geneva, and the nature and origin of these oscillations could be determined scientifically. The period of a seiche is calculated based on Merian's formula, established in 1828, which determines the period of the standing waves in an enclosed rectangular basin of constant depth:

Where:

T: Oscillation period

L: Length of the lake

g: acceleration of gravity

h : Average depth of the lake

n: Integer

By analogy, the word seiche is also used to describe the slow oscillations of sea level observed in harbours and enclosed bays. These marine seiches are often masked by other events such as tides, waves or surges. Contrary to seiches in lakes, marine seiches have not been routinely studied until recently.

Definition

Seiches correspond to oscillatory motion of the water level. They are therefore gravity waves, like ocean waves or tides. What distinguishes seiches from other types of waves is the period.

The graph above shows the distribution of wave energy on the ocean surface as a function of the period.

Seiches are gravity waves with a period that is longer than that of swells and shorter than that of semi-diurnal tides. The period of a seiche is generally between an interval of 100 seconds to a few hours.

Resonance

Seiche waves generally have a lower amplitude than seiches themselves, which is amplified by resonance.

Bodies of water in contact with the ocean are excited by incident seiche waves. Seiche waves exercise forced oscillations, which can lead to resonance if the frequency matches the frequency specific to the zone. The wave penetrates into the semi-enclosed space and is trapped. This forms a standing wave generated by the superposition of the incident wave and the wave reflected from the wall.

The diagrams below describe a standing wave in an enclosed basin of length L and constant depth h.

If λ is the wavelength, then L = n.λ

Case n=1 is called the fundamental oscillation. The water level at the node (point N) is constant over time, then it undergoes the maximum variation at the antinode (point A).

The other cases are distinguished by the number of nodes, which increases as the wavelength decreases.

How seiche waves are generated

Many events can cause seiche waves, including landslides or earthquakes. In this case, the wave is called a tsunami.

Other explanations are also frequently suggested:

Variations in precipitation intensity

Internal waves

Internal waves are oscillations on the thermocline. They may lead to changes in the sea surface level.

Irregular swells

Wind

For example, it has been shown that large variations in wind speed caused by convection cells are the source of most of the seiches in the port of Rotterdam.

Changes in atmospheric pressure

This is the most common explanation in the case of seiches in harbours. A low pressure system accompanied by changes in atmospheric pressure directly affects the sea level and can cause the formation of seiche waves.

Why worry about seiches

Seiches sometimes pose problems for the safety of navigation. To our knowledge the ports for which seiche problems have been reported are: Dunkirk, Dieppe, Port-Tudy, Royan, Bayonne, Marseille and Sète.

The main problems caused are:

Problems with water control gates

This is a problem in Cherbourg, where the automatic water control gates operate based on the water level. Rapid oscillation can cause the gates to malfunction.

Problems docking

Rapid variations in sea level cause problems for ships at dock and place severe strain on moorings. The ships may crash into each other and against the dock. At Bayonne, tugs may be required to move ships affected by seiches.

Strong currents

In Port-Tudy, seiches of up to 1.5 m generate strong currents of up to 10 knots. The current begins to present problems for ships in the event of seiches 70 cm and higher, which occurs several times per year. In the event of seiches of 1 m or more, the port is closed. This happens several times per year.

Modification of the bathymetry

Currents induced by seiches can erode certain areas of the port.

Bottoming of vessels

The seiche changes the available depth of water, and therefore affects the margin of safety for vessels vis-à-vis their draft.

Extreme levels

Extreme levels are calculated based on the statistical analysis of tide gauge data, which do not take into account seiches (filtered). The calculated levels used for risk studies of coastal flooding underestimate actual levels in case of seiches.